The present invention relates to lasers. In particular, the present invention relates to smoothing out variations in the intensity of a laser beam so that it will uniformly illuminate an image plane.
Light in the visible, near ultraviolet, and ultraviolet range is useful for such applications as laser pantogography and microlithography. Lasers are the most intense source of light, but they fail to provide a beam with uniform intensity across its cross section. This causes problems when forming eutectics or developing photoresist, because a uniform beam is important for both predicting and reducing the time required for these process steps.
There are two problems that are associated with reducing variations in beam intensity. The first is that laser beams have relatively long coherence lengths, which results in diffraction effects that produce intensity variations across the beam. The second is that the beams themselves are rarely, if ever, of uniform spatial intensity.
One solution to this problem has been proposed by Obenschain et al. in U.S. Pat. No. 4,521,075 dated Jun. 4, 1985. This patent describes a stepped lens with step heights great enough to separate the portions of the beam coming through different steps by at least their coherence length. The various portions of the beam are then recombined with a focusing lens to provide a more uniform intensity distribution. The necessary step height is at least .DELTA..sub.c /(n-1), where .DELTA..sub.c is the coherence length and n is the index of refraction of the lens material. Unfortunately, lasers having wavelengths suitable for high resolution microlithography or pantogography generally have coherence lengths that would require large, expensive, and lossy lenses to satisfy the above conditions.
Another more practical and less lossy solution to this problem has been proposed by Fan et al. In U.S. Pat. No. 4,744,615 dated May 17, 1988, and assigned to I.B.M. Corporation. This patent describes a "homogenizer"--usually made of glass, this is an internally reflective light tunnel with a polygonal cross section that breaks the beam up into an array of apparent light sources emanating from different positions. The geometry of the homogenizer has the effect of causing different parts of the beam to travel different distances while inside the homogenizer. Upon exiting the homogenizer, the output beam is more incoherent and consequently has more uniform spatial intensity. To achieve a truly high degree of incoherence the maximum path length difference from the various "sources" to the image plane must differ by at least the coherence length of the original beam. For this to happen the following two conditions must be satisfied: EQU W .sub.min =.DELTA..sub.c [R+(1+R.sup.2).sup.1/2 ]&gt;2R.DELTA..sub.c EQU R.sub.min =cot(.phi.),
Here W.sub.min is the minimum required width of the homogenizer channel, R=L/W (length divided by width) is the aspect ratio of the channel, and .DELTA..sub.c is the coherence length of the beam. The angle .phi. is the largest angle of an input array with respect to the optical axis that will be accepted by the tunnel.
If the above conditions are not satisfied, the beam sections from the different "sources" will not have path length differences greater than the coherence length, and the beams will tend to recombine coherently with a resultant nonuniform spatial intensity at the exit aperture of the homogenizer. Again, many potentially useful lasers for microlithography and pantogography have such long coherence lengths that these limitations are also too restrictive. This has been a failing to this type of device for some time.
For instance, the homogenizer conditions can only be easily be met when .DELTA..sub.c is smaller than about 3 mm. For Ar.sup.+ or bandwidth narrowed eximer lasers with the coherence lengths closer to 30 mm, the manufacturing costs of suitable homogenizers and imaging optics becomes very prohibitive because of the large optics that are necessary to achieve a 30 millimeter path length difference. There thus exists a need to relax these restrictive guidelines. One method of doing this is to reduce the apparent coherence length of the homogenizer input beam so the size of the homogenizer and its accompanying optics may be reduced.